Unraveling the chemical structures and sources of biomass-derived organic aerosols through a year-long offline analysis in Hyytiälä, Finland

Abstract. Biomass-burning OA (BBOA) and biogenic secondary OA (BSOA), both originating from biomass but from different pathways, still lack comprehensive and quantitative understanding, which limits assessments of their environmental impacts. In this study, source-resolved OAs including BBOA and BSOA in a European boreal forest were characterized by the offline use of an aerosol mass spectrometer (AMS), with improved chemical resolution offered by polarity-based fractionation. OA extract solutions were prepared according to polarity as high-polarity water-soluble organic matter, humic-like substances, and water-insoluble organic matter, and their abundances and chemical structures were analyzed by off-line high-resolution AMS analysis. Quantitative analysis revealed an annual OA concentration of 1.24 ± 0.75 µg m⁻³, with lower concentrations in winter and higher in summer. A 5-factor source apportionment solution was obtained from positive matrix factorization (PMF) of the mass spectra of the three fractions. CHN-family ions were found to be indicative of BBOA, whereas C₅H₈O₅⁺, C₅H₆O⁺, and C₈H₉O₄⁺ were identified as potential tracers for BSOA; they lead the identification of BBOA- and BSOA-like factors. CROA factor, related to aged fossil fuel combustion and aged biomass material combustion, was also identified. Different PMF factors exhibited differences in water solubility, with relatively water-insoluble characteristics of compounds containing CROA aromatic structures. This study highlights the usefulness of polarity-resolved factor analysis in understanding diverse OA sources and opens the door for the characterization of climate- and air-quality-related properties of BBOA and BSOA.